The recovery of oil or gas from natural occurrence in underground porous formations can be enhanced in a number of ways using injection fluids. Some injection fluids are forced into the production well to increase porosity of the surrounding formation, either by fracturing or acidizing the formation immediately surrounding the well. Oil recovery can also be enhanced by water flooding in which a salt solution, forced into an injection well, permeates the oil bearing formation and flows ultimately through a production well carrying with it the residual oil from the formation. Completion fluids are frequently used to maximize the recovery of oil from existing reservoirs. Such completion fluids are placed across a production zone during completion or workover of a well in order to maintain hole stability and control subsurface pressures.
In injection fluids, whether for water flooding, well completion, fracturing or acidizing, in order to minimize the flow and the loss of the fluid down cracks in the rock formation, it is desirable to add a viscosifier. A commonly used viscosifier is guar or carboxylmethyl cellulose (CMC), but in the search for improved viscosifiers, polymeric materials have found increasing favor because they can be tailored to be hydrolytically stable under the conditions that prevail in the formation, including elevated temperatures.
As described in our copending application Ser. No. 164,158, a viscosifier which has shown many advantages and is particularly stable under the conditions that prevail in underground oil producing formations, is a high molecular weight poly(vinylamine) or its acid salt. This polymer has advantages over standard polymers used as viscosifiers. For example, polyacrylamide, which rapidly hydrolyzes under acid or base conditions to give polyacrylic acid functionality, precipitates due to crosslinking by polyvalent cations (calcium, magnesium, aluminum). Guar gum or xanthan gum, which are natural polysaccharides also used as viscosifiers, undergo rapid hydrolysis to low molecular weight species at elevated temperatures, particularly in acid conditions. Poly(vinylamine), on the other hand, especially the high molecular weight polymer, shows good thermal stability in strong acid or base conditions while serving as a viscosifier. This polymer, however, when available as a dry powder, because of its high molecular weight, dissolves slowly and only with continuous agitation and, preferably, heat to give a dilute solution acceptable for acid or alkaline flooding or other injection fluid use. These dissolution steps are time consuming and add greatly to the cost of using the poly(vinylamine) as a viscosifier in injection fluids. If dissolved at the well, mixing equipment and storage vessels are required on site. On the other hand, if the dissolution is performed at a plant where such equipment exists, then large volumes of the solution must be transported to the field thereby contributing to increased cost.
Poly(vinylamine) can be prepared by polymerizing the vinylamide in water or preferably in a water-in-oil inverse emulsion to provide a high solids content emulsion. In order to maintain a reasonable viscosity, the polymer is then hydrolyzed under acid or base conditions as a very dilute solution in water. The polymer can then be acidified to give the acid salt and precipitated from water with several additional volumes of a solvent such as methanol The precipitated polymer, collected as a stringy solid, is ground to a powder and then transported to its point of use.
U.S. Pat. No. 4,623,699 (1986) discloses a method of hydrolyzing poly-(N-vinylformamide) powder at elevated temperatures with gaseous hydrogen chloride or hydrogen bromide. The poly(vinylamines) prepared are said to be useful, for example, as flocculants for wastewaters and sludges, or as retention agents, drainage agents and as flocculants in papermaking. This process, however, produces incomplete polymer hydrolysis, especially if the molecular weight of the polymer is high, as is desired for use as a viscosifier in injection fluids for enhanced oil recovery. This process also requires use of expensive equipment designed to minimize acid halide corrosion and has long process cycles.
U.S. Pat. No. 4,444,667 (1984) also discloses the use of poly(vinylamines) as a flocculating agent in sludges where the poly(vinylamine) is prepared by the hydrolysis of from 10 to 90% of the formyl groups in a poly-(N-vinylformamide).
U.S. Pat. No. 4,699,722 (1987) describes well workover fluids which contain a polymer viscosifier, such as a polymer of dimethylaminopropylmethacrylamide and copolymers which contain not over 50% comonomer.
European Patent Application Publ. No. 0120592 discloses stabilizing fines in permeable subterranean formations with certain organic polycationic polymers containing two quarternary ammonium moieties in the repeating unit.
U.S. Pat. No. 4,217,214 (1980) discloses that poly(vinylamine) hydrochloride is useful as a flocculating agent in wastewater systems.
U.S. Pat. No. 4,500,437 (1985) discloses acrylamide copolymers and terpolymers containing N-vinylformamide and N-vinylacetamide which are useful as friction reducers in acid stimulation of oil or gas wells. These polymers are said to have molecular weights ranging from 20,000 to 15.times.10.sup.6 and can be introduced into the acid solution for fracture-acidizing as an oil-in water or water-in-oil emulsion. In Examples 67-70, the polymers are prepared by inverse emulsion polymerization with the polymers of Examples 68 and 70 having molecular weights well below 100,000. Example 20 shows the preparation of poly(N-vinylformamide) by solution polymerization.
None of the above cited references address the problems associated with the dissolution of poly(vinylamine) viscosifiers of high molecular weight when it is intended to use such polymers in injection fluids for enhanced oil or gas recovery.